1. Field of the Invention
The invention relates to the field of arc discharge devices, and in particular to x-ray tubes. More specifically, the invention relates to materials for anodes for such devices.
2. Related Art
Relevant portions of a conventional X-ray tube are illustrated in FIG. 1. For simplicity, various other portions of a conventional tube, such as seals, external materials, and magnet are not illustrated. The tube has a cathode 101, a filament 102 within the cathode 101, an accelerating anode 103, a drift tube 104 and a target anode 105. Electrons burning off the filament 102 are accelerated through the accelerating anode 103, drift through the drift tube 104 to strike the target anode 105. As a consequence, the target anode produces x-rays 106. The minimum voltage needed to achieve the preferred current varies as a function of the distance d 107 between the cathode 101 and the accelerating anode 103.
It is desirable to minimize the minimum voltage required. However, when the distance d 107 becomes too small, a high current arc discharge develops between the cathode 101 and the accelerating anode 103. Such discharges eventually can destroy the accelerating anode.
Another prior art high current arc discharge device is a high voltage vacuum switch. These are used at substations of power transmission lines. These switches often develop arcing between metal contacts when switches are closed or opened.
Another prior art high current arc discharge device is a spark gap. These devices break down in response to a critical voltage in order to arrest high voltage discharges. The spark gap breaks down by arcing. A similar device is the triggered spark gap which breaks down in response to a trigger rather than in response to a high voltage discharge.
It is desirable in all of these devices to decrease arcing damage in the anodes, or electrodes acting as anodes, in order to increase the lives of the devices. If such arcing damage is reduced the distance between electrodes can be reduced. As a result, a greater range of electric field intensities can be used, and the size of the devices can be reduced. For instance the distance d 107 can be reduced.
To reduce arcing damage in high current arc discharge devices, the anodes, or electrodes acting as anodes, have traditionally been made of materials which maximize the product of melting point, specific heat and density. Traditionally this maximum was achieved with tungsten.
To reduce chemical damage in light bulb electrodes, a tungsten/rhenium alloy with small amounts of rhenium has been used, see e.g. U.S. Pat. No. 4,864,191.
To reduce mechanical damage in target anodes of x-ray tubes, tungsten/rhenium alloys have been used, see e.g. H. Cross Co., "Rhenium and Rhenium Alloys", (Weehawken N.J. 07087). It is noted that target anodes in x-ray tubes are not exposed to high current arc discharge, unlike the accelerating anodes.